The article presents data on research into the influence of a mineral-vitamin premix on the clinical condition, erythropoiesis and the metabolism (macro- and micro-elemental, vitaminic, proteinaceous, enzymic) of Hucul breed mares in the last trimester of pregnancy. The Marmix premix contains: vitamins – А, D3, Е, В1, В2, В12, pantothenic and ascorbic acids, niacin, biotin; nonorganic compounds of microelements – sulphate of cobalt, copper, zinc, iron, manganese; potassium iodide, sodium selenite; amino acids – lysine, threonine, methionine.The usage of the mineral-vitaminic premix Marmix on pregnant mares during 60 days causes recovery of clinical status, erythropoiesis, raises the level of the cobalt (by 45.9%) and copper (by 2.15 times), normalizes the calcium-phosphorus ratio, raises the phosphorus level in the blood (by 17.7%), raises vitamin А (by 2.5 times) and tocopherol (by 2.02 times), total protein (by 27.7%), decreases the level of urea (by 42.2%), activity of AspAT [aspartate transaminase or aspartate aminotransferase] (by 42.9%) and AlAT [alanine transaminase] (by 44.9%) and alkaline phosphatase (by 43.7%). The research conducted contributes to improvements in the study of the pathogenesis of microelementosis of mares and develops an efficient way of treating and preventing the development of microelementosis among these animals.